In recent years, there have been proposed and developed various interlock decision technologies in which it is determined based on a deceleration rate of a vehicle and a relation between a transmission ratio intended by a ratio-change instruction and an actual transmission ratio of an automatic transmission whether or not interlock occurs during non-shift operation of the transmission. One such interlock decision technology has been disclosed in Japanese Patent Provisional Publication No. 2008-232355 (hereinafter is referred to as “JP2008-232355”). However, the interlock decision technology disclosed in JP2008-232355, suffers from the drawback that it is impossible to detect interlock occurring during vehicle standstill. This is because a vehicle deceleration rate and a transmission ratio do not change during vehicle standstill, regardless of the presence or absence of interlock. Therefore, it would be desirable to detect interlock occurring during vehicle standstill.